Cast Whitemane Lion by paying 0 and 2 life due to Defiler of Faith and Oketra's Monument.
Defiler of Faith and Oketra's Monument trigger.
Resolve the Defiler of Faith trigger, creating a 1/1 Soldier creature token.
When the Soldier enters, Essence Warden triggers, causing you to gain life.
Resolve the Oketra's Monument trigger, creating a 1/1 Warrior creature token.
When the Warrior enters, Essence Warden triggers, causing you to gain life.
Resolve Whitemane Lion. When it enters, it and Essence Warden trigger.
Resolve the Whitemane Lion trigger, returning it from the battlefield to your hand.
Resolve the Essence Warden trigger, causing you to gain life.
Repeat.
This loop turns Whitemane Lion into a self-sustaining engine by stacking cost reducers: Defiler of Faith and Oketra's Monument work together to slash the lion's mana cost down to zero, allowing you to cast it repeatedly while paying life instead. Every single time the cat enters and bounces itself back to your hand, both monuments reward you with a token, fueling an endless cascade of creature triggers. Meanwhile, Essence Warden watches the parade and conveniently converts every single incoming token and lion into a steady stream of life, ensuring you never run out of the resource Defiler of Faith demands.